This invention relates to a coating characterized by low thermal absorption, high thermal emittance and high electrical conductivity. More particularly, this invention relates to a coating characterized by the above-stated properties which comprises a fired oxide pigment and which is useful for coating the surface of spacecraft and similar objects.
One of the requirements for long-term operation of space vehicles is the provision of a thermal control coating which will avoid charge build-up due to ionic or electric fields such as the "Solar Wind" and radiation belts. Such a coating should provide control and selectivity of optical absorptance and emittance properties and provide stability of these properties to exposure by UV radiation as well as to exposure by various ionizing radiations such as electrons, protons, X-rays, and cosmic rays which might be encountered in space environments.
Previously used thermal control coating systems used on spacecraft include organic paints, inorganic paints, polished metal surfaces, evaporated metals and oxides, and second surface mirrors. Of these, all are non-conductive except the metals and evaporated metals. Such coating systems have been found unsuitable for most parts of spacecraft because of their low emittance values which result in unacceptably high operating temperatures.
In addition to these coating systems, metallic cages have been used in some cases to shield a sensitive component from electrical fields. However, such cages not only are expensive because they must be custom fabricated, but also are unwieldy, add excessive weight, and are limited in thermal control properties.
One approach to spacecraft thermal control is to use a silicate-treated ZnO. A coating composition of this type is prepared by a method which includes treating ZnO with an aqueous alkali metal silicate solution and combining the treated pigment with a degradation-resistant binder such as a silicone polymer. The silicate treatment was found to render the ZnO pigment resistant to degradation of reflective properties upon exposure to UV radiation in a vacuum. In another approach the zinc oxide is rendered shock-resistant by heat-treatment at a temperature of 600.degree.-900.degree. C.
None of these prior art coatings, however, provide an electrically conductive coating which meets the requirements outlined above. However, as a result of these deficiencies, the present inventor began a study to discover a thermal control coating having the properties described above.
One attempted route was to make available paint systems conductive by the addition of metallic powders. However, the amount of a metallic additive is limited by the required physical properties of the paint, such as flow or spray qualities and the drying, curing and adhesion properties. As a result, it was not possible to meet the requirements for conductivity. Also, reproducibility of electrical and optical properties was found to be poor.